Journal of Crystal Growth 227–228 (2001) 138–142

Strong room-temperature exciton–photon couplingin low-finesse microcavities grown

by molecular-beam epitaxy

J.M. Suna, Y.F. Zhanga, Y.J. Hana,*, W.X. Wanga, C.L. Baoa, W. Lia,J.M. Zhoua, Qi Huanga, B.H. Fengb, X.L. Zhangb

aGroup 605, MBE Labortory, Institute of Physics, Chinese Academy of Sciences, Beijing, 100080, People’s Republic of ChinabOptical Physics Laboratory, Institute of Physics, Chinese Academy of Sciences, Beijing, 100080, People’s Republic of China

Abstract

Room-temperature strong exciton–photon coupling phenomena was studied in a low-finesse quantum microcavity

entirely filled with 17.5 pairs of GaAs (80 A)/Al0.3Ga0.7As (42 A) quantum wells. The front and back distributed Braggreflectors of the microcavity consist of only 6 and 8 pairs of l=4 stacks of GaAs (30 A)/AlAs (5 A) superlattices andAlAs layers. And the l=4 GaAs (30 A)/AlAs (5 A) superlattices are equal to the Al0.2Ga0.8As layer in the distributed

Bragg reflector. Large Rabi splitting of 9.4meV was observed at resonance with heavy-hole excitons at roomtemperature. Photoluminescence spectra showed a transition from linear regime to high carrier density nonlinear regimedue to loss of oscillator strength and collapse of the coupling at high excitation intensity. # 2001 Published by Elsevier

Science B.V.

PACS: 42.50.�p; 42.60.Da; 42.65.Pc

Keywords: A3. Molecular beam epitaxy; A3. Quantum wells

1. Introduction

Semiconductor quantum microcavities (QMCs)have attracted significant attentions in the past fewyears. QMCs provided a full solid structure forcontrolling the spontaneous emission of quantumwell excitons as well as the interaction betweenlight and excitons [1–3]. Numerous attempts onthe former leads to the demonstration of low-threshold vertical cavity surface emitting laser

(VCSELs) operating in a wide wavelength rangedfrom mid-infrared to the blue region [4–6]. Thedevelopment of the latter were concentrated on theexperimental and theoretical studies on the strongexciton–photon coupling regime between light andexciton in QMCs, where the eigenmodes of thecoupling system were two exciton–photon mixedstates named cavity polaritons with a spectralseparation called vacuum Rabi splitting [7–17].The value of Rabi splitting, which reflected thecoupling strength between quantum well excitonwavefunction with electromagnetic field, is animportant parameter in strong coupling regime

*Corresponding author.

E-mail address: mbe@aphy.iphy.ac.cn (Y.J. Han).

0022-0248/01/$ - see front matter # 2001 Published by Elsevier Science B.V.

PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 6 5 2 - 2

of QMCs. For clear observation of a well-resolvedRabi splitting of cavity–exciton polariton, Rabi-splitting should be larger than the linewidth ofexciton states. This condition is easily fulfilled atlow temperature for high-finesse QMCs containinga small number (3–5) of quantum wells where thelinewidth of the cavity and exciton states aresmaller than the value of Rabi splitting. Withincreasing temperature, thermal broadening of theexciton linewidth caused a significant reduction ofthe Rabi splitting. This is the reason why only afew reports involved Rabi-splitting of cavitypolaritons at room temperature [18–20]. Increas-ing as much as possible the value of Rabi splittingis important for study of the strong exciton–photon coupling regime and its application atroom temperature. Since the Rabi splitting isproportional to the square root of the excitonoscillator strength multiplied by the overlap of theexciton wavefunction with the electromagneticfield, there are two aspects for increasing the valueof Rabi splitting. One attempt was increasing theelectromagnetic field confined in the cavity bymeans of improving the refractive index contrastin distributed Bragg reflectors [18,19]. Anothereffective method was including as much as possiblethe number of quantum wells [20].

2. Experimental procedure and results

In this work, we studied the strong couplingphenomena in a 1l GaAs QMC designed forresonance at room temperature. Contrary to thepreviously reported structures, the entire 1l cavitywas filled with 17.5 pairs of GaAs/AlGaAsquantum wells. This allows observation of a largeRabi splitting due to sufficient overlap of thequantum well exciton wavefunction with electro-magnetic field inside the cavity. Although increas-ing the number of quantum wells might cause areduction of Rabi splitting due to the absorptionbroadening of the cavity mode, this influence wasweaker than the contribution to the increase ofRabi splitting when the width of cavity mode wascomparable to the exciton linewidth. Strongcoupling phenomena still could be observed atroom temperature in a low-finesse cavity with a

small number of distributed Bragg reflectors.Reduction of the distributed Bragg reflectors havemany advantages in the growth of QMCs, such assignificantly reducing the growth time and im-prove the reduplication of the structures.

The microstructures used in this work weregrown by molecular beam epitaxy (MBE) inVG80H system with one aluminum source. Ahome-made laser reflectivity system was used tocalibrate the growth rate and the thickness of thedistributed Bragg reflectors. The distributed Braggreflectors were quarter-wave stacks of Al0.2-Ga0.8As/AlAs layers where the Al0.2Ga0.8As layerwas replaced by GaAs (30 A)/AlAs (7 A) super-lattices having a equivalent refractive indexdetermined by the reflectivity spectrum. This couldreduce 2–3 times of the growth time for thedistributed reflectors in the case of one aluminumsource in the MBE system. The entire cavity layerconsists of 17.5 pairs of GaAs (80 A)/Al0.3Ga0.7As(42 A) quantum wells. The front and back mirrorsconsist of 9 and 6 pairs of quarter wave stacks ofGaAs (30 A)/AlAs (7 A) superlattices/A1As layerswith reflectivity of 0.93 and 0.82 at center of thereflectivity band. The calculated finesse of themicrocavity was 20. Two similar microcavitieswere grown for resonance with heavy-hole ex-citons at nearly perpendicular incidence andincident angle around 288 at room temperature.Reflectivity spectral measurements have beenperformed at room temperature using a tungstenlamp and a Fourier transforms infrared interfe-rometer. The signal was detected using a cooledGe detector. Various detuning between the cavitymode and the exciton can be probed by changingthe angle of the incident light. Temperaturedetuning was also performed with a temperaturevariation from 77 to 300K.

Fig. 1 shows the reflectivity spectra measured atdifferent incident angle y with respect to thegrowth direction of the structure. The cavity modeshifts to high energy with increase of the incidentangle. This allows tuning of the relative position ofthe cavity mode and the excitons. The spectrameasured at incident angle of 108, 288, 448, and 608are for negative detuning, resonance with heavy-hole exciton, resonance with light-hole exciton andpositive detuning. Large Rabi splitting up to

J.M. Sun et al. / Journal of Crystal Growth 227–228 (2001) 138–142 139

9.4meV was observed at incident angle of 288 atresonance with heavy-hole exciton. To the best ofour knowledge, this is the largest value of room-temperature Rabi splitting that has been observedfor GaAs/AlGaAs quantum well cavity withstandard AlGaAs/AlAs distributed Bragg reflec-tor. This large splitting was attributed to sufficientoverlap of the quantum well exciton wavefunctionwith the electromagnetic fields in the cavity. Abarely resolved splitting of 5.5meV was alsoobserved at resonance with the light-hole exciton.

The reflectivity spectra were measured at differ-ent temperatures from 77 to 300K. The angle ofincident light was set to 458 in order to observeboth of the coupling phenomena at resonance withlight and heavy excitons. Fig. 2 shows the reflec-tivity spectra measured at 77, 208, 240 and 300K.Rabi splitting of 9.8meV was observed at 208K atresonance with heavy-hole exciton. It reduced to7.3meV at resonance with the light-hole exciton at240K.

Room-temperature Rabi splitting is also ob-served in the photoluminescence (PL) spectra

excited by the 514 line of an Argon laser atdifferent relative excitation light power and Fig. 3shows the results. The data were taken for the caseof cavity-exciton detuning of 1meV. Low excita-tion power shows typical double-peaked emissionof cavity polaritons. The PL spectral shows anonlinear behavior as increasing the excitationpowers. A cross-over behavior of the polaritonmodes was observed where the intensity of thehigh-energy cavity-like polariton increased fasterthan the exciton like polariton at low energy. A redshift of the exciton-like polariton was alsoobserved due to an increase of temperature athigh excitation intensity. To our knowledge, this isthe first observation of the transition of a QMCfrom linear regime to a nonlinear, high carrierdensity regime at room temperature, where the PLshows loss of oscillator strength, collapse of thecoupling. Following the detailed study reported byKhitrova et al. [10,21] at low temperature, thisnonlinear behavior was explained as the excitonabsorption saturation which simply saturated thePL intensity of the exciton like polariton at highexcitation intensity.

The cavity–polariton transmission spectrumwas measured at room temperature after removing the substrate as shown in Fig. 4. Themultiple interference analysis gives the cavitytransmission as a function of the energy for

Fig. 1. Room-temperature reflectivity spectra measured at

different incident angles.

Fig. 2. Reflectivity spectra of the microcavity measured at

different temperatures for positive detuning, resonance with

heavy-hole excitons, light-hole excitons and negative detuning.

The spectra were measured at an incident angle of 458.

J.M. Sun et al. / Journal of Crystal Growth 227–228 (2001) 138–142140

axial propagation as [16]

TðnÞ ¼ T1T2e�al

ð1� R1R2e�alÞ2 þ 4R1R2e�alsin2e=2; ð1Þ

where R ¼ffiffiffiffiffiffiffiffiffiffiffi

R1R2

p(R1, R2, T1 and T2 is the

reflectivity and transmission of the front and backmirrors), al is the single pass absorption (where l is

the total length of the quantum wells), e is theround trip phase shift which depends on thereflectivity index n and the cavity-free spectralrange DFSR ¼ c=2Leff (where Leff is the effectivecavity optical length, c is the speed of light)according to, e¼ 2pðn� noÞ=DFSRþ4pðn�1Þln=cwhere no is the frequency of the exciton absorptionpeak. The heavy- and light-hole exciton states aretreated as two independent Lorentz oscillatorswhere the oscillating strength of the light-holeexcitons are 2.5 times weaker than that of theheavy-hole excitons [23,24]. The calculated finesseof the cavity was 20 for R1 ¼ 0:82, R2 ¼ 0:90. Thetheoretical simulation at resonance of the heavy-hole exciton was shown as the dashed curve inFig. 4 for aol ¼ 0:49 and much broad excitonlinewidth dH ¼ 22 meV. From the theoreticalsimulation, we obtained the average absorptioncoefficient at heavy-hole exciton was3.4� 104 cm�1. This value is consistent with thereported results (3–4� 104 cm�1) [22].

3. Conclusions

In summary, we demonstrated that strongcoupling phenomena still could be observed in alow-finesse (as low as 20) QMCs entirely filled withsemiconductor quantum wells. Large Rabi split-ting above 9.4meV was observed at resonancewith heavy-hole excitons by temperature andangle-tuning techniques. PL spectra showed atransition from linear regime to high carrierdensity nonlinear regime due to loss of oscillatorstrength, collapse of the coupling in the QMCs.These results indicated that this structure might beused as optoelectronic devices at room tempera-ture.

Acknowledgements

This work is supported by Chinese NationalFoundation of Natural Sciences under ContractNo. 69896260 and Key Project of ChineseAcademy of Sciences under Contract No. KJ951-A1-405. One of us would like to acknow-ledge the assistant of Mrs. Gong Nan in the

Fig. 3. PL spectra of the microcavity at different excitation

power.

Fig. 4. Transmission spectrum at resonance with the heavy-

hole exciton and theoretical simulation. The solid curve is

experimental result and the dotted curve is theoretical simula-

tion with a0¼ 4:3� 104 cm�1

J.M. Sun et al. / Journal of Crystal Growth 227–228 (2001) 138–142 141

Materials growth and Dr. M.H. Zhang for usefuldiscussion.

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